The present invention relates in general to ceramic cutting tool materials and, in particular, to such materials in which monocrystalline silicon carbide whiskers or platelets are homogeneously distributed in an oxide based matrix and, more particularly, to such composites where the crystal structure of the silicon carbide whiskers is modified during sintering from a low to a high hexagonal to cubic ratio.
Ceramic cutting tools have been available for several decades. It is not until the last decade that they have achieved a more substantial importance for use in chip forming machining. The main reason for the limited acceptance of ceramic cutting tools has been sudden and unexpected tool insert failures because of inadequate strength, toughness and thermal shock resistance.
During the last ten years, the properties of ceramic cutting materials have been improved in many respects which is why they have especially increased their relative share in the cutting of cast iron and heat resistant alloys.
Aluminum oxide based cutting tools are very sensitive to thermal cracking as aluminum oxide itself has a relatively low thermal conductivity. This fact leads to very short tool lives under conditions leading to thermal shock, i.e., high cutting edge temperatures, short engagement times and varying cutting depths.
To a certain extent, the thermal properties of aluminum oxide have been improved by the addition of titanium carbide and/or titanium nitride which improves the thermal conductivity of the composite material. Their addition also increases the hardness of the composite. In comparison with pure aluminum oxide materials, an increased tool life is therefore obtained in cutting of hard workpiece materials and in operations demanding high resistance to thermal shocks. This type of material has, however, too poor a toughness behavior for a more general use and is therefore mainly used at relatively low feeds.
Another, later step of development includes the addition of homogeneously dispersed fine-grained zirconium oxide particles in a matrix of aluminum oxide. This type of material is described in the U.S. Pat. No. 4,218,253. Transformation of the "metastable" zirconium oxide particles during use increases both strength and toughness and thus will lead to a more predictable tool life. The thermal properties of said material are, however, insignificantly better than those of pure aluminum oxide materials which is why initiation and growth of thermally induced cracks still create problems in practical cutting operations generating high cutting edge temperatures.
It has more recently been shown that addition of silicon carbide whiskers, monocrystalline hair crystals, to an aluminum oxide matrix leads to a greatly improved fracture toughness and strength. This type of material is described in U.S. Pat. No. 4,543,345. Ceramic materials based upon this concept have proved a very good performance in cutting of heat resistant alloys, especially Inconel 718.
Zirconium oxide and silicon carbide whiskers can also be combined as described in the U.S. Pat. No. 4,657,877.
Both the mentioned additions have led to substantial improvements of the toughness behavior in certain metal cutting operations. However, the search for greater improvements of the properties of cutting tools is continuously ongoing.
The properties of silicon carbide whisker reinforced ceramic composites are to a great extent dependent on their processing, since the chemistry of the whisker surface will determine the strength of the bond between the whisker and the matrix. A strong chemical bond is to be avoided since the desired bridging and deflecting properties of the whiskers then is lost. This is further described in the U.S. Pat. No. 4,916,092.
Silicon carbide exists in a number of different polytypes (Ramsdell LS, Amer. Miner. 32, p. 64-82, 1947). These different polytypes have different stability areas (Zangvil and Ruh, J. Am. Cer. Soc. 71, 10, p. 884-90, 1980). Possible mechanisms for transformation from cubic to hexagonal silicon carbide includes deformation and diffusion (Kravets VA, Izvestiya Akademii Nauk SSSR, Neorganicheskie Materialy, 20, n 4, pp. 694-696, 1984). It is known that it is possible to change the crystal structure of silicon carbide single crystals per se from cubic to hexagonal by heating. The transformation may also be influenced by the chemical composition of the whisker.
Commercially available silicon carbide whiskers have a cubic or a mixed cubic to hexagonal crystal structure having a low amount of the hexagonal crystal structure, generally about 20% hexagonal, remainder cubic crystal structure. During sintering of a silicon carbide whisker reinforced aluminum oxide, it has now been found possible to alter the original crystal structure of the whisker even while embedded in the aluminum oxide matrix and to partially transform the cubic crystal structure to a hexagonal crystal structure. The transformation is favored by a high sintering temperature.